ceph-disk-prepare: verify device is not mounted before using

Make sure the data and/or journal device(s) are not in use (mounted)
before using them.  Make room for additional "in-use" checks in the future.

Closes: #3256
Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk-prepare: verify device is not in use by device-mapper

Be nice and tell the user which devices/mappings are consuming the device,
too.

Signed-off-by: Sage Weil <sage@inktank.com>